Motion picture films are part of our cultural heritage. Unfortunately, they are often affected by undesirable objects such as scratches, dust, dirt, stains, abrasion and some more. These usually come from the technical process of developing, handling, storing, and screening or scanning the film footage. In some rare cases static objects may already be induced during capturing, for example fluff within a lens or dirt on a scanner glass. In the following all these defects will be simply referred to as ‘scratch and dirt’.
Scratch and dirt reduce the quality and the joy of film watching. This is, in most cases, not because a significant amount of visual information is lost. Actually, the opposite is true, scratch and dirt provide an extra amount of information, although it is entirely irrelevant. The human brain is very effective at substituting missing relevant information, based on its daily experience of vision. But its experience of how physical things in this world usually look and behave is constantly violated when viewing affected footage. For example, dirt deposited on the film during scanning or screening appears in one frame and disappears in the other. This contradicts human experience about objects usually appearing, moving, and disappearing smoothly and causes the brain to treat the abnormal visual stimulus exceptionally. Separating relevant and irrelevant information in this way is therefore stressful and tiring for the viewer. Removal of scratch and dirt is required to keep viewers relaxed and focused on the content and is, therefore, an important part of any restoration process.
Preferably restoration is carried out digitally after scanning. Although there are wet scanning techniques, which feed the material through a chemical cleansing process to reduce dirt and potential scratches before scanning, they are not used widely due to a number of technical problems which make them quite costly. In addition, a lot of footage has already been scanned and archived digitally on tape.
Apparently manual restoration of digitized films by finding and removing each scratch and dirt object is a time consuming business, although there is software on the market that assists artists in many aspects of the job. In particular, manual restoration of old content with large amounts of either scratch or dirt may not be financially viable. This is even more the case for large archives with footage of unknown commercial value.
The application of automatic restoration software with algorithms that try to detect and remove scratch and dirt is the only viable alternative to a manual process. At present there are a number of software and hardware products available on the market which perform detection and removal of scratch and dirt more or less automatically. Usually a manual adjustment of certain parameters is needed to fine tune detection and removal, sometimes individually for each scene. After processing, the restored output or parts thereof have to be either accepted or rejected, with the option of rerunning restoration with different parameters. This is unsatisfactory since the adjustment takes time and quality may be not good enough in critical scenes that have not been specially adapted. Detection of scratch and dirt is a nontrivial problem that is currently not totally solved. There is still a certain ratio of objects that will either not be detected or falsely detected.